The present invention relates to an improvement of the internal combustion engines of the type in which a relatively lean fuel-air mixture is burnt in order to reduce the contents of the harmful emissions in the exhaust gases.
There has been devised and demonstrated various methods of burning the lean fuel-air mixture, that is, the mixture with a high air-fuel ratio, in internal combustion engines in order to reduce the harmful emissions such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) from the exhaust gases. However, the ignitability of the lean fuel-air mixture is, in general, poor, which is especially true when residual gases remain in the vicinity of an ignition plug. Furthermore, the combustion speed of the lean mixture is low, resulting in a poor thermal efficiency of the engine and a high fuel consumption rate (g/horsepower.sup.. time) when applied in an automotive engine.
In order to overcome the poor ignitability mentioned above, there have been devised and demonstrated the stratified charge type internal combustion engines in which, in order to ignite the lean fuel-air mixture, a zone of relatively rich mixture, which may be easily ignited, is formed around the spark plug, which is initially ignited by the spark plug and the resulting flame ignites the lean fuel-air mixture.
There have been also proposed internal combustion engines to overcome the low combustion speed, which is provided with an auxiliary combustion chamber of the type in which the main and auxiliary combustion chambers are intercommunicated through a passage. The fuel-air mixture pushed into the auxiliary combustion chamber through the passage during the compression stroke is first ignited, and the resulting combustion flame with both high temperature and pressure is injected from the auxiliary combustion chamber through the passage into the main combustion chamber to ignite the lean fuel-air mixture in the main combustion chamber. The passage intercommunicating between the main and auxiliary combustion chambers is, in general, narrow so that the combustion flame is discharged through the passage into the main combustion chamber at considerably high speeds, by which large portions of the lean mixture contained in the main combustion chamber may be ignited at once. Moreover, intensive turbulence occurs in the main combustion chamber due to the injection of the combustion flame from the auxiliary combustion chamber, by which the speed of combustion in the main combustion chamber can be further improved. This is known as the "torch effect."
In order to improve both the ignitability and combustion speed, that is, to combine the effectiveness of the internal combustion engines of the types described above, there have been devised and demonstrated the stratified charge type internal combustion engine with an auxiliary combustion chamber. In this internal combustion engine, there is provided an auxiliary combustion chamber communicated with a main combustion chamber through a passage. A lean mixture is charged into the main combustion chamber through a main intake valve while the easy-to-ignite rich mixture is charged into the auxiliary combustion chamber through an auxiliary intake valve or formed by the injection of the fuel through a fuel injection valve or nozzle. The relatively rich mixture in the auxiliary combustion chamber is first ignited by the ignition plug so that the combusting flame may be injected through the passage into the main combustion chamber, by which the lean fuel-air mixture in the main combustion chamber is ignited. Therefore, the internal combustion engines of this type is very effective for the combustion of the lean air-fuel mixture both in its ignitability and combustion speed. However, this engine has very distinct defects in that the fuel supply system is quite complicate because the rich and lean mixtures must be charged separately. For instance, the rich mixture is charged into the auxiliary combustion chamber from the auxiliary carburetor through an auxiliary intake valve. Therefore, three valves that is, the main intake valve, the auxiliary intake valve, and the exhaust valve must be mounted on one cylinder head. Since the intake systems of both the rich and lean mixtures must be provided separately, the construction of the cylinder head itself becomes very complex, and therefore difficult casting of the cylinder heads is required, resulting in poor productivity and increased cost. Furthermore the main carburetor for the lean mixture and the auxiliary carburetor for the rich mixture must be so controlled that a predetermined relation between the rich and lean mixtures must be correctly maintained or varied depending upon the operating conditions of the engine. Therefore, the members of, for instance, the linkage interconnecting the throttle valves of the main and auxiliary carburetors must be made with a higher degree of tolerance. Otherwise, the precise and simultaneous control on the main and auxiliary carburetors required on this engine becomes unobtainable. The fuel injection valve used to provide the rich mixture in the auxiliary combustion chamber usually causes the construction to become more complex, thus resulting in further increase in its cost.